Among the conventional hydraulic circuits for turning a crane, there is known a circuit which, as shown in FIG. 1, is designed to supply a pressurized discharge oil off a hydraulic pump 1 to a turning-purpose hydraulic motor 3 via a directional switching valve 2. The directional switching valve 2 may be a directional switching valve of center-bypassing type in which a spool 6 is fittedly inserted into a spool bore 5 of a valve body 4 as shown in FIG. 2.
In this hydraulic circuit, the spool 6, when lying at a neutral position as shown in FIG. 2, is adapted to unload the pressurized oil from the pump 1 via a center-bypassing passage 7 to a tank 8, while closing between a first and a second actuator port 9 and 10 connected to the turning-purpose hydraulic motor 3 on the one hand and a tank port 11 on the other hand to halt the turning movement of a turning body. When the spool 6 is slidably moved by the operator for switching the directional switching valve 2, the center-bypassing passage 7 is throttled to increase the pump pressure while opening between a turning pump port 12 and the first or second actuator port 9 or 10. If the pump pressure is raised above the drive pressure for turning, the pressurized oil is caused to thrust a check valve (a valve for preventing a reversed flow) 13 open to flow and the return oil from the turning-purpose hydraulic motor 3 flows to the tank 8 after passing between one of the second and the first actuator port 10 and 9, that are opening simultaneously, and the tank port 11. Accordingly, it follows that the turning-purpose hydraulic motor 3 is caused to rotate with a rate of the flow that results by deducing from the pump discharge quantity the flow quantity that is bled off into the tank 8 from the center-bypassing passage 7.
As a matter of course, if the spool 6 is further slidably moved to a full stroke end to completely close the center-bypassing passage 7, the turning-purpose hydraulic motor 3 is rotated by using a full quantity of the pump discharge.
Also, when the hydraulic motor 3 is to be stopped, it is braked by slidably moving the spool 6 in the opposite direction to throttle the opening between the afore-mentioned second or first actuator port 10 or 9 and the tank port 11. The motor 3 is reduced in speed by setting the pressurized oil supplied from the hydraulic pump 1 free into the center-bypassing passage 7. The turning-purpose hydraulic motor 3 is finally stopped by closing between the second or the first actuator port 10 or 9 and the tank port 11.
In this connection, it should be noted that the opening between the afore-mentioned second or first actuator port 10 or 9 and the tank port 11 and the opening of the directional switching valve for the center-bypassing passage 7 vary oppositely to each other.
As in the foregoing, the rate of flow controlled when the turning-purpose hydraulic motor 3 is accelerated is determined depending on how much the quantity of flow from the center-bypassing passage 7 is set free into the tank 8. In other words, when the opening of the center-bypassing passage is larger, the rate of flow into the turning-purpose hydraulic motor 3 is less. And, when the opening of the center-bypassing passage 7 is smaller, the rate of flow into the turning-purpose hydraulic motor 3 is increased.
However, the rate of flow into the turning-purpose hydraulic motor 3 if the pump discharge rate is small is made equal to that in which the quantity set free from the center-bypassing passage 7 is increased if the pump discharge rate is large. Accordingly, it is necessary to further throttle (lessen) the opening of the center-bypassing passage 7 in order that the turning-purpose hydraulic motor 3 may be rotated up to the number of rotations identical to where the pump discharge rate is large. Therefore, the relationship between the spool position and the turning velocity is as represented by the graph in FIG. 3 indicating that at the beginning of flow (motion), the spool position is varied according to the rate of discharge of the pump.
This means that when a suspended load is turned by an operator's operation, the point (the spool position) at which the motion of it is initiated depends upon the number of rotations of an engine connected to the hydraulic pump 1 and thus the point is not constant. Therefore, the operator, while looking for the point at which the movement begins by operating the lever which is provided to control the spool, must find the point of commencement of the motion and then proceed with a gradual acceleration.
Also, when the turning body is to be braked, the turning-purpose hydraulic motor is rotated by the inertia of the turning body including the suspended load, and the braking pressure is increased to reduce the speed by throttling the opening between the second or first actuator port 10 or 9 and the tank port 11 for reducing the return flow quantity. Therefore, because of the need to adjust the braking pressure in accordance with the magnitude of the inertia, the opening between the afore-mentioned second or first actuator port 10 or 9 and the tank port 11 must be adjusted while depending on the experience of the operator.
Accordingly, in case, too, where the turning body is being braked, the operation magnitude of the lever stroke must be adjusted to control the turning velocity in accordance with the weight of the suspended load and the size of the turning radius. The relationship between the spool position and the turning velocity in this case is as represented by the graph of FIG. 4.
When the suspended load is being slightly turned and then stopped by an operator's operation, the opening off the center-bypassing passage 7 is slightly throttled to drive the turning-purpose hydraulic motor 3 and immediately thereafter the return opening, i.e. the opening between the second or first actuator port 10 or 9 and the tank port 11 must be throttled to brake the turning-purpose hydraulic motor 3. However, because the conventional turning-purpose hydraulic motor has the characteristic at the time of acceleration and that at the time of the deceleration which are respectively as shown in the graphs of FIGS. 3 and 4, when the rotation speed of the engine is so slow that the discharge rate of the pump may be small, the difference between the spool position having the characteristic at the time of acceleration and the spool position having the characteristic at the time of deceleration which are as shown in the graph of FIG. 5 is enlarged so that even when an attempt is made by the operator to slightly move and then stop the suspended load by returning the lever at its neutral position, the stoppage of the suspended load is significantly delayed, resulting in an excessive movement thereof and an extreme difficulty by the operator to handle it.
Further, if the turning body is slightly inclined, it is susceptible of receiving the influence of the gravity; and if the boom is long, it is susceptible of receiving the influence of a wind. Thus, even when an attempt is made by the operator to slowly turn the turning body, it tends to accelerate along a downward inclination or to be blown in the wind; hence the turning speed tends to be increased in opposition to the operator's intention.
Also, as disclosed in Japanese Unexamined Patent Publication No. SHO53-21379 there is known a hydraulic circuit which is made capable of controlling the torque and the velocity at the time of acceleration according to the magnitude of the operating magnitude of a pilot valve by using a direction control valve for supplying a pressurized oil to an hydraulic motor, switching the said direction control valve among a first pressurized oil supply position, a neutral position and a second pressurized oil supply position depending on the pilot pressure of the pilot valve and making a relief valve variably settable in accordance with the pilot pressure.
This hydraulic circuit being, however, of the type in which the direction control valve at its neutral position communicates the pump discharge passage and the actuator circuit with the tank, when the hydraulic motor is to be stopped from the state in which it is driven while the direction control valve is taking the first pressurized oil supply position, it is necessary to forcibly stop the hydraulic motor by operating the direction control valve to the second pressurized oil supply position over the neutral position. Hence, if the hydraulic motor is used for turning a crane, the crane cannot be stopped by being accurately positioned.
Further, as disclosed in Japanese Unexamined Patent Publication No. SHO56-80507, there is known a hydraulic circuit which includes a direction control valve for supplying a hydraulic motor with a pressurized discharge oil from a hydraulic pump, in which the direction control valve is switched among a neutral position and a first and a second pressurized oil supply position according to the pilot pressure from a pilot valve and which is provided in the discharge passage of the hydraulic pump with a relief valve whose setting pressure is variable depending on the pilot pressure.
In this hydraulic circuit, there is an interruption between an actuator port and a tank port when the direction control valve lies at the neutral position. Thus, by varying the amount of the slidable movement of the spool towards the neutral position of the direction control valve, the opening areas of the actuator port and the tank port are varied to restrict the return oil flow from the hydraulic motor. This enables the stopping speed of the hydraulic motor to be controlled. However, when tills hydraulic motor is used for turning a crane, if the load or the inertia are varied according to the weight of a suspended load or the like, the rate of passing flow is varied while the afore-mentioned opening areas are made constant. Thus, the slopping speed is made so irregular that the stopping may occur in front of a target position or there may be an overreach before the stopping.
Accordingly, it is an object of the present invention to provide a turning-purpose hydraulic circuit which is capable of acceleration and deceleration in an identical pattern without regard to the weight of a suspended load or the size of a turning radius; eliminates the need for an operator's adjustment regardless of a variation in the speed under the influence of a wind or the inclination of a turning body; and is capable of being controlled by even a beginner at will.
The present inventor has discovered what is described below upon various investigations of crane-turning operations.
More specifically, it has been found that like a crane-turning operation, where the load to a hydraulic motor is small and the acceleration pressure is small, the acceleration must proceed smoothly, and at the time of acceleration, a torque control by the pressure control off a hydraulic motor is more rational than a velocity control by the flow control thereof. In this case, however, since a great initial torque is required at the time of motion commencement, the flow rate must be controlled in such a manner that the pressure is first increased and then an acceleration follows when the movement is commenced while gradually reducing the pressure. When a stationary velocity is reached, the pressure of a magnitude necessary to maintain it must remain applied while the flow rate is controlled. At the time of stopping, it must be done before a target is reached. The operator, while watching the remaining distance and the current velocity, will have to reduce the velocity until the stoppage is reached. When the turning inertia is great, a beginner tends not to reduce the velocity much as he/she is expected and, immediately before the target stop, tends to suddenly reduce the velocity with the consequence of largely jolting the suspended load. Accordingly, for the operator, it is rational to perform the control of the flow rate coupled with a pressure compensation. If so performed, the operation of turning a body independent of the magnitude of the inertia is made possible.